Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace that has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products including olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules. The pyrolysis process, however, produces molecules that tend to combine to form high molecular weight materials known as tar. Tar is a high-boiling point, viscous, reactive material that can foul equipment under certain conditions. In general, feedstocks containing higher boiling materials tend to produce greater quantities of tar.
The formation of tar after the pyrolysis effluent leaves the steam cracking furnace can be minimized by rapidly reducing the temperature of the effluent exiting the pyrolysis unit to a level at which the tar-forming reactions are greatly slowed. This cooling, achieved in one or more steps and using one or more methods, is referred to as quenching. Quenching may be supplemented upstream of the quenching step by pre-quenching hot effluent with a pre-quenching oil. This reduces the process gas to a somewhat lower temperature, e.g., typically by less than about 100° F. (55° C.) cooler, at which temperature cracking can still occur.
Conventional steam cracking systems have been effective for cracking high-quality feedstock which contains a large fraction of light volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost heavy feedstocks such as, by way of non-limiting examples, crude oil and atmospheric residue. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of about 1100° F. (593° C.) otherwise known as resids. The non-volatile components of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized.
In most commercial naphtha and gas oil crackers, cooling of the effluent from the cracking furnace is normally achieved using a system of transfer line heat exchangers, a primary fractionator, and a water quench tower or indirect condenser. The steam generated in transfer line exchangers can be used to drive large steam turbines which power the major compressors used elsewhere in the ethylene production unit. To obtain high energy-efficiency and power production in the steam turbines, it is necessary to superheat the steam produced in the transfer line exchangers.
Cracking heavier feeds, such as kerosenes and gas oils, produces large amounts of tar, which lead to fouling in the transfer line exchangers preferred in lighter liquid cracking service.
Additionally, during transport some naphthas are contaminated with heavy crude oil containing non-volatile components. Conventional pyrolysis furnaces do not have the flexibility to process residues, crudes, or many residue or crude contaminated gas oils or naphthas which are contaminated with non-volatile components.
To address coking problems, U.S. Pat. No. 3,617,493, which is incorporated herein by reference in its entirety, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 450° F. and 1100° F. (232° C. and 593° C.). The vapors are cracked in the pyrolysis furnace into olefins and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709, which is incorporated herein by reference in its entirety, discloses a process to minimize coke deposition. It describes preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize about 50% of the heavy feedstock with superheated steam and the removal of the residual, separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking. Periodic regeneration above pyrolysis temperature is effected with air and steam.
U.S. Pat. No. 5,580,443, which is incorporated herein by reference in its entirety, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a gas-liquid separator to separate and remove a required proportion of the non-volatiles as liquid from the separator. The separated vapor from the gas-liquid separator is returned to the pyrolysis furnace for heating and cracking.
The bottoms obtained from a vapor/liquid separator or flash drum are typically suited for use only as relatively low value fuel oils, with no significant utility within the cracker itself. In view of the low value of such fuel oils versus intial feed values, it would be desirable to utilize the vapor/liquid separator bottoms in a higher value application, preferably within the overall cracking process itself.
Using heavy or residual materials, such as bottoms, as quench oils in steam cracking operations is known. U.S. Pat. No. 3,923,921 to Kohfeldt, incorporated herein by reference in its entirety, discloses quenching naphtha steam cracking effluent using a high boiling fraction, i.e., tar bottoms and a steam-cracked gas oil fraction, which fractions are recovered from the quench tower. U.S. Pat. No. 4,233,137 to Ozaki et al., incorporated herein by reference in its entirety, teaches treating hot cracked cracking effluent with a low temperature hydrocarbon oil of high thermal stability (e.g., an oil enriched with aromatics) which can be a by-product heavy oil from the thermal cracking or supplied from an outside source. U.S. Pat. No. 4,663,019 to Gartside et al., incorporated herein by reference in its entirety, teaches thermally cracking a side cut fraction from a fractionation tower fed with residual oil, and quenching the cracked product with residual oil. Coke produced from the quenched product provides reaction heat for the cracking.
EP 0 911 378 B1, incorporated herein by reference in its entirety, teaches reducing viscosity of circulating quench oil in a mixture of partially cooled cracker effluent by separating the resulting vapor and liquid, withdrawing vapor to a fractionator, and withdrawing liquid as a heavy, tarry, fuel oil product, which improves viscosity of the circulating oil.
Various injection nozzle designs for use in quenching cracked product streams are known. U.S. Pat. No. 3,593,968 to Geddes, incorporated herein by reference in its entirety, discloses cooling pyrolysis gases flowing downwardly through a quench zone whose walls are covered by a film of quench oil while spraying quench oil into the gas stream. The quench oil is directed through an annular opening comprising a frustoconical element. U.S. Pat. No. 3,959,420 to Geddes et al., incorporated herein by reference in its entirety, discloses a device wherein the entry of the quench chamber is in the form of an inverted frustum with a curved transition configuration between the inverted frustum and the straight wall of the quench chamber. Quench oil is added through a reservoir. U.S. Pat. No. 4,121,908 to Raab et al., incorporated herein by reference in its entirety, discloses an apparatus for cooling a cracking gas steam having a nozzle for introducing cooling oil mounted at the elbow of a cracking gas pipe (see FIG. 3).
U.S. Pat. No. 4,384,160 to Skraba, incorporated herein by reference in its entirety, discloses pre-quenching cracked effluents of ethane/propane feeds using hydrocarbons such as liquid ethane and/or propane.
It would be desirable to provide an apparatus and process for cracking feeds, including feeds that contain resids, which utilize bottoms obtained from a vapor/liquid separator used to treat resid-containing feeds prior to cracking. In particular, it would be advantageous to utilize such bottoms in higher value applications, rather than as fuels. Although the bottoms obtained from a vapor/liquid separator typically contain crackable components, they also contain asphaltenes, rendering these bottoms unfit for use as steam cracking feed. Moreover, the bottoms are not suited as quench oils because they contain paraffins, which, if uncracked, would cause precipitation of tar asphaltenes downstream in the transfer line and primary fractionator.